Experimental study on mean velocity and turbulence characteristics of plane Couette flow: low-Reynolds-number effects and large longitudinal vortical structure

Mean-velocity and turbulence measurements have been made in the turbulent near wake of a flat plate at various Reynolds numbers in order to investigate the low-Reynolds-number effects in this region. The results indicate that the low-Reynolds-number effects are significant enough to partially explain the discrepancies in the existing mean-velocity data. It has been found that, while the Reynolds-number-independent, inner-law similarity of the boundary layers continues to exist, the width of the inner wake that develops within the inner-law region scales with the outer variable. Therefore, the mean velocity near the wake centreline depends on the Reynolds number. It is conjectured that this is due to the influence of the large eddies of the outer layer on the spreading of the inner wake.Measured turbulence quantities indicate that sudden changes occurring just downstream of the trailing edge are independent of the Reynolds number, but the subsequent development of the turbulent stress profiles depends on the Reynolds number. The Reynolds shear stress and the mean-velocity profiles within the inner wake show approximate similarity.

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Bursts and Low-Reynolds-Number Effects in Turbulent Boundary Layers

Fluid Mechanics and Its Applications - Advances in Turbulence V ◽

10.1007/978-94-011-0457-9_100 ◽

1995 ◽

pp. 549-553

Author(s):

P. S. Westbury ◽

J. F. Morrison

Keyword(s):

Reynolds Number ◽

Boundary Layers ◽

Low Reynolds Number ◽

Turbulent Boundary Layers ◽

Reynolds Number Effects ◽

Low Reynolds

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Shear Layer Development, Separation, and Stability Over a Low-Reynolds Number Airfoil

Journal of Fluids Engineering ◽

10.1115/1.4039233 ◽

2018 ◽

Vol 140 (7) ◽

Cited By ~ 4

Author(s):

Paul Ziadé ◽

Mark A. Feero ◽

Philippe Lavoie ◽

Pierre E. Sullivan

Keyword(s):

Boundary Layer ◽

Reynolds Number ◽

Shear Layer ◽

Separation Bubble ◽

Low Reynolds Number ◽

Base Flow ◽

Mean Velocity ◽

Laminar Separation ◽

Low Reynolds ◽

Layer Development

The shear layer development for a NACA 0025 airfoil at a low Reynolds number was investigated experimentally and numerically using large eddy simulation (LES). Two angles of attack (AOAs) were considered: 5 deg and 12 deg. Experiments and numerics confirm that two flow regimes are present. The first regime, present for an angle-of-attack of 5 deg, exhibits boundary layer reattachment with formation of a laminar separation bubble. The second regime consists of boundary layer separation without reattachment. Linear stability analysis (LSA) of mean velocity profiles is shown to provide adequate agreement between measured and computed growth rates. The stability equations exhibit significant sensitivity to variations in the base flow. This highlights that caution must be applied when experimental or computational uncertainties are present, particularly when performing comparisons. LSA suggests that the first regime is characterized by high frequency instabilities with low spatial growth, whereas the second regime experiences low frequency instabilities with more rapid growth. Spectral analysis confirms the dominance of a central frequency in the laminar separation region of the shear layer, and the importance of nonlinear interactions with harmonics in the transition process.

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Unsteady swimming of small organisms

Journal of Fluid Mechanics ◽

10.1017/jfm.2012.177 ◽

2012 ◽

Vol 702 ◽

pp. 286-297 ◽

Cited By ~ 40

Author(s):

S. Wang ◽

A. M. Ardekani

Keyword(s):

Reynolds Number ◽

Low Reynolds Number ◽

Fundamental Equation ◽

Added Mass ◽

Natural Environments ◽

Mean Velocity ◽

Unsteady Forces ◽

Uniform Background ◽

Low Reynolds

AbstractSmall planktonic organisms ubiquitously display unsteady or impulsive motion to attack a prey or escape a predator in natural environments. Despite this, the role of unsteady forces such as history and added mass forces on the low-Reynolds-number propulsion of small organisms, e.g. Paramecium, is poorly understood. In this paper, we derive the fundamental equation of motion for an organism swimming by means of the surface distortion in a non-uniform background flow field at a low-Reynolds-number regime. We show that the history and added mass forces are important as the product of Reynolds number and Strouhal number increases above unity. Our results for an unsteady squirmer show that unsteady inertial effects can lead to a non-zero mean velocity for the cases with zero streaming parameters, which have zero mean velocity in the absence of inertia.

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